You do know you could alight at Milton Keynes avoid the whole town mess, cross platform and get to East Croydon on the Southern service, from their same platform every few minutes down to Gatwick .
it’s pretty painless . However why are you so interested in using a London Terminal from Manchester anyway .
Vast majority indeed far MORE European routes originate quite locally and COVID aside prime US cities were serviced frequently and frankly anywhere else go via Amsterdam .
If it’s a sort of national loyalty to an airline branded British again why its a Pan- European business with the largest single investors being an Qatari sheikh!